SRS Labs SR560 - Manual (Page 30)

APPENDIX B
B-2
ways which these noise sources work their
way into an experiment.
Capacitive Coupling
A voltage on a nearby piece of apparatus
(or operator) can couple to a detector via a
stray capacitance. Although C
stray
may be
very small, the coupled in noise may still be
larger than a weak experimental signal.
Capacitive Noise Coupling
To estimate the noise current through C
stray
into the detector we have
I = C
stray
dV = jwc
stray
V
noise
dt
Where a reasonable approximation to C
stray
can be made by treating it as parallel plate
capacitor. Here, w is the radian frequency of
the noise source (perhaps 2
∗ π ∗ 60 Hz),
V
noise
is the noise voltage source amplitude
(perhaps 120 VAC). For an area of A =
(0.01 m)
2 and a distance of d = 0.1 m, the
‘capacitor’ will have a value of 0.009 pF and
the resulting noise current will be 400 pA.
This meager current is about 4000 times
larger than the most sensitive current scale
that is available on the SR510 lock-in.
Cures for capacitive coupling of noise
signals include:
1) Remove or turn off the interfering noise
source.
2) Measure voltages with low impedance
sources and measure currents with high
impedance sources to reduce the effect
of i
stray
.
3) Install capacitive shielding by placing
both the experiment and the detector in
a metal box.
Inductive Noise Coupling
Inductive Coupling
Here noise couples to the experiment via a
magnetic field:
A changing current in a nearby circuit gives
rise to a changing magnetic field which
induces an emf in the loop connecting the
detector to the experiment, (emf = d
Ø
B
/dt).
This is like a transformer, with the
experiment-detector loop as the secondary
winding.
Cures for inductively coupled noise include:
1) Remove or turn off the interfering noise
source (difficult to do if the noise is a
broadcast station).
2) Reduce the area of the pick-up loop by
using twisted pairs or coaxial cables, or
even twisting the 2 coaxial cables used
in differential hookups.
3) Use magnetic shielding to prevent the
magnetic field from inducing an emf (at
high frequencies a simple metal
enclosure is adequate).
4) Measure currents, not voltages, from
high impedance experiments.